The present invention relates to a method for the photochemical decomposition of a volatile chlorine-containing organic compound in a vent gas. More particularly, the invention relates to a method for the photochemical decomposition of a volatile chlorine-containing organic compound, such as trichloroethylene and tetrachloroethylene, contained in a vent gas or exhaust air, which may cause a serious problem of environmental pollution.
Volatile organic chlorine compounds such as trichloroethylene and tetrachloroethylene are used in large quantities as a degreasing solvent in dry cleaning and washing solvent in various processes of metal-working and semiconductor industries. These organic chlorine compounds are notoriously carcinogenic so that one of the most serious problems in respect of environmental pollution in recent years is the contamination of atmospheric air and drinking water by these compounds as the contaminant or pollutant. Despite the utmost care in the above mentioned industrial processes for preventing dissipation of these compounds, namely, it is almost unavoidable that a considerable amount of these chlorine compounds employed in the process is emitted to the environmental atmosphere due to their high volatility. Moreover, the purifying processes of water for city water supply or for in-plant utility and sawage disposal sometimes involve a step of aeration of the raw or waste water so that, when the water under aeration is contaminated with these volatile organic chlorine compounds, the volatile compounds are readily transferred into the vent air. Therefore, such a vent air or gas must be discharged out of the site after removing or decomposing the volatile organic chlorine compounds into a harmless or less harmful form.
Various proposals and attempts have been made in the prior art for removing or decomposing such a volatile organic chlorine compound contained in a vent gas, mainly, by using an adsorbent. For example, Japanese Patent Kokai No. 62-163730 proposes a method in which a plural number of adsorption columns are installed and run in an alternately cycling steps of adsorption of the chlorine compounds on the adsorbent and regeneration of the saturated adsorbent while the vent gas containing the volatile organic chlorine compounds is under ultraviolet irradiation with a germicidal and ozone-generating light. Further, Japanese Patent Kokai No. 61-153190 proposes use of a modified synthetic zeolite as the adsorbent.
Though relatively reliable, the above mentioned adsorption methods have problems and disadvantages in respect of the limited adsorption capacity of the adsorbent or, in particular, extremely low adsorption capacity of the adsorbent for a constituent contained in the vent gas in a low concentration and troublesome running procedure of the process including a treatment for the regeneration of the adsorbent. Moreover, the irradiation with an ozone-generating ultraviolet light is economically disadvantageous due to the high cost for the lamps.